Reland: Make ScrollbarThemeAura selectively invalidate scrollbar parts.
In particular:
- when hovering or pressing the thumb, repaint only the thumb
- when thumb position changes, repaint the track only if it changes
(e.g. due to a button being disabled upon reaching the end)
BUG=
549277
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_oilpan_rel,mac_blink_oilpan_rel
Committed:
https://crrev.com/54342454be5d73c64aa47b57facd880ba8df2b36
Cr-Commit-Position: refs/heads/master@{#367105}